1. Field of the Invention
The present invention relates to a road friction coefficients estimating apparatus for a vehicle for estimating friction coefficients on road surfaces and more particularly to a road friction coefficients estimating apparatus capable of accurately estimating friction coefficients on road surfaces by a simple method using vehicle motion parameters such as lateral acceleration, yaw rate and the like.
2. Discussion of Prior Arts
In recent years, numerous vehicle control technologies such as traction control technologies, braking force control technologies, torque distribution control technologies and the like, have been proposed and some of these control technologies have been realized in actual automobile markets. Many of these control technologies use friction coefficients on road surfaces (hereinafter, referred to as xe2x80x9croad friction coefficientxe2x80x9d) for calculation or correction of control parameters. Accordingly, in order to execute the control properly, it is necessary to estimate accurate road friction coefficients.
Several technologies in which road friction coefficients are estimated based on vehicle motion parameters such as lateral acceleration, yaw rate have been proposed. For example, the applicant of the present invention proposes a technology in which road friction coefficients are estimated based on the comparison of an actual yaw rate estimated from an observer with a yaw rate calculated using a vehicle motion model on a high friction coefficient road surface and a yaw rate calculated using a vehicle motion model on a low friction coefficient road surface respectively in Japanese Patent Application No. Toku-Gan-Hei 11-217508.
However, since the above technology needs two vehicle motion models, high and low friction coefficient road surface models, the technology has a disadvantage of taking much time for tuning. Particularly, in case of the low friction coefficient road surface model, it is necessary to take a nonlinearity of tire into consideration and therefore the vehicle motion model becomes complicated and this is a primary cause of taking much time for tuning. Further, since the above technology is constituted by two vehicle motion models, the technology has a defect of complicated logic and large amount of calculations.
It is an object of the present invention to provide a vehicular road friction coefficient estimating apparatus having a simple construction and a small amount of calculations and capable of easily making a tuning and estimating stable and accurate road friction coefficients over broad traveling conditions.
The road friction coefficient estimating apparatus for a vehicle comprises an actual value estimating means for estimating an actual value of a vehicle motion parameter, a high friction coefficient road reference value estimating means for estimating a high friction coefficient road reference value of the vehicle motion parameter based on a vehicle motion model when the vehicle travels on a road surface with high friction coefficient and a road friction coefficient estimating means for forming a Lissajou""s figure based on the actual value and the high friction coefficient road reference value and for estimating a road friction coefficient according to an area of the Lissajou""s figure when a gradient of the Lissajou""s figure is in the neighborhood of 45 degrees and for estimating a road friction coefficient according to a lateral acceleration of the vehicle when the gradient is in a range out of the neighborhood of 45 degrees.